Apparatus for monitoring and controlling the concentration of toner in a developer mix

ABSTRACT

Samples of a developer mix, comprising toner and magnetic particles, are passed continuously through a coil connected in an oscillator circuit. The inductance of the coil changes with variations in the concentration of the toner in the samples of the developer mix, and, consequently, the circuit provides output signals that are related to the concentrations of toner in the samples. Means responsive to the output signals control the addition of replenisher toner to the developer mix as toner is consumed from the developer mix in a developing process.

United States Patent Inventors Henderson C. Gillespie Moorestown; I Ralph Herman, Cherry Hill; Milton M. Sowiak, Trenton, NJ.

Appl. No. 811,132

Filed Mar. 27, 1969 Patented Mar. 30, I971 Assignee RCA Corporation APPARATUS FOR MONITORING AND CONTROLLING THE CONCENTRATION OF TONER IN A DEVELOPER MIX 6 Claims, 5 Drawing Figs.

U.S. Cl 222/56,

Int. Cl B67d 5/08 Field of Search 222/57, 55, I, 76, 56

d iomsr SOURCE [56] References Cited UNITED STATES PATENTS 2,909,303 10/ l 959 Henderson 222/57 3,255,921 6/1966 Eveson et al 222/55X 3,399,652 9/1968 Gawron ZZZ/57X Primary Examiner-Stanley H. Tollberg Assistant Examiner-David A. Scherbel Attorney-Glenn H. Bruestle ABSTRACT: Samples of a developer mix, comprising toner and magnetic particles, are passed continuously through a coil connected in an oscillator circuit. The inductance of the coil changes with variations in the concentration of the toner in the samples of the developer mix, and, consequently, the circuit provides output signals that are related to the concentrations of toner in the samples. Means responsive to the output signals control the addition of replenisher toner to the developer mix as toner is consumed from the developer mix in a developing process.

Patented March 30, 1971 2 Sheets-Sheet 1 -6 VOLTAGE SOURCE Fig.2.

INVENTORS Henderson 6. 6/7/esp/5 Ralph Herman, a

Milton M. Sow/bk.

BY fl/iZb/J ITTORNEY arransrus Eon MONITORING AND CONTROLLING ran coNcEnrnArloN or TONER n: A DEvELorEn BACKGROUND OF INVENTION This invention relates to apparatus and a method for monitoring and controlling the concentration of toner in a developer mix. More particularly, the invention relates to apparatus and a method for monitoring and controlling the concentration of toner in a developer mix comprising pigmented toner particles and carrier particles of magnetic material to provide a triboelectric effect between the toner particles and the carrier particles. The apparatus and process of the present invention are particularly useful in electrostatic-printing systems wherein electrostatic charge patterns are rendered visible by application thereto of a triboelectric developer mix of pigmented toner particles and magnetic particles, such as iron filings or iron powder.

In many prior art electrostatic-printing systems wherein electrostatic charge patterns on an insulating surface are developed by a triboelectric developer mix of toner and magnetic particles, only the toner particles are consumed in the developing process and the magnetic particles remain and are reused. Consequently, it is necessary to replenish the developer mix with additional toner periodically or continuously to maintain the concentration'of toner in the developer mix within relatively narrow limits to insure developed prints of proper density. If, for example,'the concentration of toner in the developer mix is less than an optimum concentration, the density of the developed charge patterns is too light. On the other hand, it the concentration of toner in the developer mix is too high, the toner particles tend to adhere to the nonimage areas of the print, providing prints with a dirty" greyish background.

It has been proposed to monitor the concentration of toner in a developer mix by photoelectric methods, such as by measuring the light reflected from samples of developer mix, or by measuring the light reflected from developed electrostatic charge patterns, but such apparatus and methods are relatively complex, require delicate adjustments, and are affected by external conditions, such as dust, the color of the pigment toner, and the color of the recording element involved.

SUMMARY OF INVENTION The novel method of monitoring the concentration of toner in a developer mix, comprising an electrically conductive carrier, comprises the steps of (l) passing samples of the developer mix past, and adjacent, a coil connected in an AC (alternating-current) circuit, whereby the inductance of the coil varies as a function of the concentration of the toner in the samples of the mix and the AC circuit provides output signals which vary with the concentration of the toner in the samples, and (2) comparing the output signals to the reference signals of known concentrations of toner in the mix, whereby the concentration of toner in the samples is determinable.

In one embodiment of the novel method, replenisher toner is added to the developer mix in response to output signals of predetermined amplitude, maintaining the concentration of toner in the developer mix within relatively narrow limits.

In another embodiment of the novel method the output signals determine the speed with which replenisher toner is added to the developer mix.

Novel apparatus for monitoring the concentration of toner in a mass of the developer mix comprises means, disposed within the coil, for receiving the samples of the developer mix therethrough. A resonant circuit including the coil is connected in AC circuit means so that the output signals of the circuit means are a function of the inductance of the coil, as determined by the concentration of toner in the samples. The concentration of toner in the developer mix is controlled within predetermined limits by replenisher toner feed means that are operated in response to the output signals.

By the term AC circuit means," as used herein, is meant a source of fluctuating voltage that would affect the inductance of a coil in circuit with the source. Hence the AC circuit means may include a source of interrupted DC voltage.

DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic view of an electrostatic-printing system embodying the present invention;

FIG. 2 is a cross-sectional view of the developing means and the developer mix sampling means of the system, taken along the line 2-2 in FIG. I;

FIG. 3 is a schematic diagram of one embodiment of circuit means and replenisher toner means of the present invention;

FIG. 4 is a graph illustrating variations in the output voltage of the circuit means in FIG. 3 as a function of known concentrations of toner in the developer mix; and

FIG. 5 is another embodiment of AC circuit means and replenisher toner means of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I of the drawings, there is shown improved apparatus 10 for monitoring and controlling the concentration of toner in a developermix H2 in an electrostaticprinting system I4 of a type well known in the art. The electrostatic-printing system 14 comprises a web 16 of a recording element that is pulled from a roll 18, by a driven roller 20 in cooperation with a pressure roller 22, along a path including an electrostatic chargirig station 24, an image exposing station 26, a developing station 28, and a fixing station 30.

The web 16 of the recording'element comprises a relatively conductive coating of paper 32 coated with a photoeonductive layer 34, such as zinc oxide in an insulating resin binder. The surface of the photoconductivelayer 34 is electrostatically charged in darkness by a suitably energized corona discharge device 36 at the discharge station 24. The uniformly charged photoconductive layer 34 is exposed, as by an image projector 38, at the image exposing station 26, to provide an electrostatic latent image, an electrostatic charge pattern, which is developed at the developing station 28 by a magnetic brush of the type to be hereinafter described. The developed image is fixed, as by a heater 40 at the fixing station 30, in a manner well known in the electrostatic-printing art.

The developer mix 12 is a homogeneous powder mixture that comprises pigmented toner particles and magnetic particles, such as iron powder or filings. The particles have an average diameter on the order to about 10 microns, and the toner is preferably fusable by heat so that it may be fixed by the heater 40 at the fixing station 30. For each electrostaticprinting system there is usually an optimum concentration of toner in the developer mix I2, depending, for example, upon the speed of development desired. The concentration of toner in the developer mix 12 is usually on the order of about 3 to 5 per cent of toner to about 97 to per cent of iron powder. Since only the toner in the developer mix i2 is consumed in the developing process, the original toner has to be replaced with replenisher toner as the original toner in the developer mix 12 is consumed.

The apparatus for monitoring and controlling the concentration of toner in the developer mix I2 will be explained in connection with a magnetic brush developing device, but it is within the contemplation of the present invention to employ developing means other than the magnetic brush type illustrated herein, such as, for example, cascade developing means. The details of the magnetic brush-developing apparatus, illustrated in FIG. I, are described in U.S. Pat. No. 3,040,704 for fApparatus for Developing Electrostatic Printing," issued to W. H. Bliss on Jun. 26, 1962, and assigned to the same assignee as that of the instant invention. Briefly, an elongated rod magnet 42 of semicircular cross section is disposed within a tube M of nonmagnetic material, such as brass. Means are provided to rotate the tube 44, in the direction of the arrow 46, about the stationary magnet 42 so that the mixture of toner and iron particles in the developer mix 12 are attracted to the surface of the tube 44 on only the side of the tube 44 adjacent the magnet 42. Thus, developer mix 12 is conveyed to the recording element 16 to develop charge patterns on it. An elongated bar 48 of soft iron is fixed to the planar surface of the rod magnet 42 to function as a magnetic shunt so that the magnetic particles already on the surface of the tube 44 will not be magnetically held on the side adjacent the magnetic-shunting bar 48, whereby the magnetic particles will fall ofl the tube 44 and return to the mass of the developer mix 12.

The developer mix 12 is disposed in a reservoir 50. A paddle-type agitator 52 is disposed within the reservoir 50 and is mechanically coupled to a motor 54 for rotation thereby. Thus, when the agitator 52 is rotated in the direction of the arrow 55, the developer mix 12 is urged against the rotating tube 44. The rotatable tube 44 has one end coupled to a shaft 56, as shown in FIG. 2, for coupling to a suitably energized motor 58. in developing electrostatic charge patterns on the recording element 16, the developer mix 12 is caused to brush against the surface of the photoconductive layer 34, whereby the triboelectrically charged toner is attracted to the electrostatic charge patterns. A developing electrode 60, adapted to be connected to a source of suitable voltage, is disposed in contact with the paper 32 of the recording element 16 adjacent the developing station 28 in a manner known in the art.

Means are provided to sample uniform quantities of developer mix 12 continuously to determine the concentration of toner therein. To this end, a sampling tube 62 is disposed beneath the reservoir 50. The upper open end of the tube 62 communicates with the reservoir 50, and the lower end of the sampling tube 62 communicates with a return tube 64 adjacent a closed lower end thereof. The upper end of the return tube 64 is open and communicates with the bottom of the reservoir 50. A screw 66 is disposed within the return tube 64 in a manner whereby a shaft 68 of the screw 66 is coupled to the motor 54 for rotation thereby. With this arrangement, sample portions of the developer mix [2 fall by gravity through the sampling tube 62 and are returned to the reservoir 50 by the rotating screw 66 within the return tube 64. By these sampling means representative samples of the developer mix are circulated continuously through the sampling tube 62.

A sampling coil 70 is disposed around the sampling tube 62 and connected in an AC circuit 72 whereby the inductance of the coil 70 is a function of the concentration of toner in the developer mix within the sampling tube 62, and wherein the rectified output of the circuit 72 varies with the inductance of the coil 70. The rectified output of the AC circuit 72 is applied to the input of a meter relay 74, and the output of the meter relay 74 is applied to a motor 76 whose shaft is coupled to a toner feed mechanism 78. The motor 76 is energized by any suitable source of electrical energy in series with the meter relay 74.

The toner feed mechanism 78 comprises means to replenish 4 the developer mix 12 with replenisher toner as toner is consumed from the developer mix 12 in a developing process. To this end, the toner feed mechanism 78 comprises a hopper 80 disposed within, and/or above, the open end of the reservoir 50. The bottom of the hopper 80 is formed with an elongated opening for receiving a slotted shaft 82 therein. The hopper 80 is filled with replenisher toner 84 similar to the toner in the developer mix. The slotted shaft 82, shown also in FIG. 5, is adapted to be controlled by the motor 76 in response to signals (output voltages) from the circuit 72, resulting from predetermined values of inductance of the coil 70 to cause the replenisher developer 84 to fall into the reservoir 50.

Referring now to FIG. 3, there is shown means for detecting changes in the inductance of the coil 70 with changes in the concentration of toner in the developer mix 12 and means to provide signals to actuate the toner feed mechanism 78 to replenish toner that has been consumed from the developer mix 12. AC circuit means, such as an oscillator 90 of fixed frequency, shown within the dashed rectangle 90, is connected to a series resonant circuit including the coil 70 and a variable capacitor 92. The AC circuit means comprises a l MHz clock 94 connected to an inverter 96 which functions as a buffer. The output of the inverter 96 is connected to the base ofa transistor 98 which, in turn, is connected as an emitter follower. The collector of the transistor 98 is connected to a suitable source of voltage and the emitter of the transistor 98 is connected to an inner conductor of a coaxial cable 100. The outer conductor of the coaxial cable 100 is grounded, and the end of the cable is terminated with a suitable termination rcsistor 102. One end of the coil 70 is connected to one end of a potentiometer resistor 104 through a rectifier I06, and the other end of the coil 70 is connected to the other end of the potentiometer resistor 104. A filter capacitor 108 is connected across the potentiometer resistor 104.

In operation, the output signals (voltages) across the potentiometer resistor 104 vary with the impedance of the coil 70 which, in turn, varies with the concentration of the toner in the sample of the developer mix 12 within the sample tube 62. The output signals across the potentiometer resistor 104 are applied to the meter relay 74 through a tap on the potentiometer resistor 104. I

The meter relay 74 is of the type sold by API Instrument Co., Cleveland, Ohio, and has a plurality of sets of contact points so that the motor 76 can be cnergimd or stopped at predetermined metcr settings resulting from the output signals of the circuit 72 The motor 76 is energized from a suitable source of energy, such as l 15 volts AC in response to predetermined meter settings within the meter relay 74 in a manner well known in the art.

The meter relay 74 is calibrated to energize the motor 76 when it is necessary to replenish the developer mix l2 with replenisher developer 84, and to dccnergizc the motor 76 when a desired amount of replenisher developer 84 has been added to the developer mix 12. With this type of operation, it is possible to maintain the concentration of toner in the developer mix 12 within predetermined limits, say, for examplc, between 3 percent and 5 percent.

The output voltage across the potentiometer resistor 104 is calibrated to correspond to known percentages of concentration of toner in the developer mix 12. Such a calibration is shown by the graph in FIG. 4. With zero concentration of toner in the developer mix 12, that is, with a developer mix comprising I00 percent iron powder, the capacitor 92 is adjusted for a maximum output voltage, 30 volts. A 3 percent concentration of toner in a sample of developer mix within the sampling tube 62 changes the inductance of the coil 70 so as to provide a rectified output voltage of about 2l volts. When the concentration of toner in .the developer mix 12 within the sampling tube 62 is 5 percent, the output voltage is about 17 volts.

Since it is desired to maintain the concentration of toner in the developer mix 12 in the range between 3 percent and 5 percent, one of the switches (not shown) in the meter relay 74 is set to close as the output signals reach I) volts, representing a concentration of 4 percent toner in the developer mix l2. Under these conditions, the motor 76 is actuated and replenisher developer 84 falls into the reservoir 50 where it is mixed with the developer mix 12 by the agitator 52. Since the new mixture of the developer mix 12 is not sampled immediately by the sampling coil 70, the output signals will increase slightly beyond 19 volts for a short time. When sufficient replenisher toner 84 has fallen into the reservoir 50, the output signals decrease, and the relay switch of the meter relay 74 opens when the output signals decrease below l9 volts. No more replenisher toner 84 is added to the developer mix 12 until enough toner is consumed form the developer mix 12 to cause the output signals to reach 19 volts again. Thus, the concentration of the toner in the developer mix 12 is maintained within a relatively narrow range. The extent of the range is determined by the time it takes for representative samples of the developer mix 12 in the reservoir 50 to reach the sampling tube 62.

An additional relay switch in the meter relay 74 can be actuated to stop the motor 54 if the concentration of the toner within the developer mix 12 falls below a predetermined amount, say about Z'Xpercent, for example, as when all of the replenisher toner 84 has been consumed. Under the latter conditions, the motor 54 can be stopped when the output signals increase to about 22.5 volts.

Referring now to FIG. 5 of the drawing, there is shown another embodiment of AC circuit means wherein the coil 70 is connected in an oscillator circuit 108 of the tuned-plate tuned-grid type adapted to produce oscillations of a frequency of about 1 MHZ. The coil 70 is connected in parallel with a capacitor 109 to provide a resonant circuit 111 that is loosely coupled to a resonant circuit 113 in the plate circuit of the oscillator circuit 108. The output signals of the oscillator circuit 108 are derived across a resistor 114 in the grid circuit and are applied to the input of an amplifier 112. The output of the amplifier 112 is applied to the motor '76 which, in turn, is coupled to the slotted shaft 82 of the toner feed mechanism 78. With the circuit shown in FIG. 5, the motor 76 is energized to rotate continuously. The speed of rotation of the motor 76 is a function of the amplitude of the output signals, and the amplitude of the output signals is a function of the impedance of the coil 70 resulting from the concentration of toner in the sampling tube 62.

In operation, the grid current through the resistor 114 depends on the state of tune of the oscillator circuit 108. When the grid circuit, which includes the I coil 70, is close to resonance with the plate circuit, the grid drive is relatively high and the grid current is also relatively high. The oscillator circuit 108 is so adjusted that this condition occurs with zero or nearly zero concentration of toner in the developer mix 12. Under these conditions output signals from the resistor 114 are amplified by the amplifier 112 andfed to the motor 76, causing replenisher toner 84 to be dispensed at the maximum rate. As the concentration of toner in the developer mix 12 rises due to this action, the inductance of the coil 70 changes so as to drive the grid portion of oscillator circuit 108 further out of tune with the plate circuit, thereby reducing the grid current of the oscillator tube. This, in turn, causes the motor 76 to run at a reduced speed and to dispense replenisher toner 84 at a slower rate. A balance is thereby achieved between the utilization rate of toner from the developer mix 12 and the dispensing rate of replenisher toner 84 from the hopper 80, whereby the concentration of toner in the developer mix 12 is maintained within a relatively small. range.

While the improved apparatus and method for monitoring and controlling the concentration of toner in a developer mix has been described with respect to a developer mix comprising toner and magnetic particles, it is within the contemplation of the present invention to employ the improved apparatus and method with developer mixes comprising toner particles and electrically conductive particles of a nonmagnetic nature if the electrically conductive particles are capable of affecting the inductance of the sampling coil through which the developer mix passes. Nonmagnetic materials affect the inductance of the sampling coil by the production of eddy currents and thereby affect the output signals of an oscillatory circuit of which the sampling coil is a part. When the carrier particles of the developer mix are nonmagnetic, the developer mix can be applied to the recording element by a brush or signals of said AC circuit means vary as a function of the concentration of toner in said samples;

means connected to said AC circuit means to indicate said output signals as a function of the concentration of said toner in said samples;

a reservoir for said mass of developer mix;

said sampling tube having one end communicating with said reservoir, whereby samples of said developer mix can fall into said sampling tube;

a return tube having one end communicating with said reservoir and an opposite end communicating with said sampling tube adjacent the other end of said sampling tube, and

means to move said samples of developer mix through said return tube to said reservoir, whereby said samples may be sampled continuously.

2. Apparatus as described in claim 1, wherein:

said means to move said samples of developer mix through said return tube to said reservoir comprising a screw disposed within said return tube; and

motive means coupled to said screw.

3. Apparatus as described in claim 1, wherein:

said AC circuit means comprising an oscillator; and

said means connecting said coil in circuit with said AC circuit means comprising a capacitor forming a resonant circuit with said coil.

4. Apparatus as described in claim 1, comprising, in addition:

replenisher toner means operative in response to said output signals to control the addition of replenisher toner in said mass, whereby to control the concentration of toner in said developer mix.

5. Apparatus as described in claim 4, wherein:

said means to indicate said output signals comprise a meter relay; and

said replenisher toner means comprise a toner feed mechanism disposed above said mass; and

a motor coupled to said mechanism and connected to be controlled by said output signals applied to said meter relay.

6. ln an electrostatic-printing system of the type wherein electrostatic charge patterns on. an insulating surface are developed by a developer mix, comprising a mixture of toner and magnetic particles, wherein only the toner particles are consumed in the developing process, and wherein replenisher toner from a toner feed mechanism is added to the developer mix to maintain the concentration of toner in the developer mix within a predetermined range, the improvement comprismg:

a capacitor and a coil;

means to pass samples of said developer mix past, and adjacent, said coil including a tube of nonmagnetic material disposed within said coil;

and oscillator;

means connecting said capacitor and said coil in a resonant circuit in circuit with said oscillator, whereby the inductance of said coil and output signals of said oscillator vary as a function of the concentration of toner in said samples;

signal indicating means;

means connecting said signal indicating means to the output of said oscillator circuit to indicate said output signals as a function of the concentration of the concentration of said toner in said samples;

said toner feed mechanism, comprising a hopper for replenisher toner, being disposed over said developer mix;

said signal indicating means comprising a meter relay; and

means connecting said meter relay to said toner feed mechanism to add said replenisher toner from said hopper to said developer mix in response to predetermined ones of said output signals. 1 

2. Apparatus as described in claim 1, wherein: said means to move said samples of developer mix through said return tube to said reservoir comprising a screw disposed within said return tube; and motive means coupled to said screw.
 3. Apparatus as described in claim 1, wherein: said AC circuit means comprising an oscillator; and said means connecting said coil in circuit with said AC circuit means comprising a capacitor forming a resonant circuit with said coil.
 4. Apparatus as described in claim 1, comprising, in addition: replenisher toner means operative in response to said output signals to control the addition of replenisher toner in said mass, whereby to control the concentration of toner in said developer mix.
 5. Apparatus as described in claim 4, wherein: said means to indicate said output signals comprise a meter relay; and said replenisher toner means comprise a toner feed mechanism disposed above said mass; and a motor coupled to said mechanism and connected to be controlled by said output signals applied to said meter relay.
 6. In an electrostatic-printing system of the type wherein electrostatic charge patterns on an insulating surface are developed by a developer mix, comprising a mixture of toner and magnetic particles, wherein only the toner particles are consumed in the developing process, and wherein replenisher toner from a toner feed mechanism is added to the developer mix to maintain the concentration of toner in the developer mix within a predetermined range, the improvement comprising: a capacitor and a coil; means to pass samples of said developer mix past, and adjacent, said coil including a tube of nonmagnetic material disposed within said coil; and oscillator; means connecting said capacitor and said coil in a resonant circuit in circuit with said oscillator, whereby the inductance of said coil and output signals of said oscillator vary as a function of the concentration of toner in said samples; signal indicating means; means connecting said signal indicating means to the output of said oscillator circuit to indicate said output signals as a function of the concentration of the concentration of said toner in said samples; said toner feed mechanism, comprising a hopper for replenisher toner, being disposed over said developer mix; said signal indicating means comprising a meter relay; and means connecting said meter relay to said toner feed mechanism to add said replenisher toner from said hopper to said developer mix in response to predetermined ones of said output signals. 